"We call for an international investigation, we can't trust a probe headed by a government henchman," said UNP general secretary Tissa Attanayake.
The call for probe came as the London-based rights group Amnesty International said that the Sri Lanka government should not be allowed to conduct its own probe.
"The Sri Lanka army should have never been policing unarmed demonstrators in the first place," Amnesty said in a statement.
Attanayake said it was a question about the level of democracy in the island as people's right to democratic protest had been challenged. "This will add a black patch to the government in the international eyes," he added.
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Residents of Weliweriya alleged that their ground water has been contaminated by acid waste, dumped by a nearby rubber gloves manufacturing plant. They have been demanding action for several weeks, leading to Thursday's police and military crackdown.
UNP leader Ranil Wickremesinghe asked for an independent probe unto the Army's action in Parliament. The Leader of the House, Nimal Siripala de Silva, assured that government would conduct an inquiry.
The Army had earlier defended its action claiming it was protecting themselves against an organised gang who had incited the residents to protest.